This invention relates to multi-axis computer numerically controlled (CNC) machines having a cutting tool that is designed to operate upon a workpiece. In the context of this invention, the term cutting tool is not restricted to standard turning or milling cutters, but also includes all mechanical, electronic and/or electromechanical devices used to modify the shape of a workpiece. Examples of cutting tools include: end-mills, turning tools, grinding wheels, laser cutting beams, plasma beams and punch tools. This invention is particularly applicable to a CNC machine in which a cutting tool, such as a grinding wheel, is moved to operate on a workpiece mounted in a rotatable chuck or headstock.
The development of multi axes and multifunction machine tools in conjunction with the development of sophisticated computer controlled operations has facilitated the emergence of a generation of very high speed precision machine tools capable of complex multi-step operations from one machine.
Such machine tools typically hold a workpiece in a controlled position whilst a movable cutting tool acts upon the workpiece to perform the desired task. The workpiece is traditionally held such that its central longitudinal axis coincides with the rotational axis of the headstock.
Typically a CNC machine is controlled by a computer program, called a part program, which serially instructs the machine to perform a sequential series of discrete operations in a predetermined sequence so that the movable operative part moves along a programmed path determined by the part program. Each individual instruction is termed a xe2x80x9cblockxe2x80x9d and many constitute a determining command for each or a combination of controllable axes. For example, a block may instruct a grinding wheel to move 5 mm in the Y axis at a given velocity. Multiple axis blocks may instruct a grinding wheel to rotate and move forward 0.05 mm in the X and Y axis at a given velocity. The blocks, once programmed into the computer, are then fixed in a set sequential order. The whole set of sequential blocks may then be automatically operated by the CNC machine which then operates from start to finish of the part program.
CNC machines conventionally include a trajectory interpolator and a position controller. The trajectory interpolator produces interpolated position commands from inputs representing a feed rate specification, i.e. All the data required to determine the desired feedrate along the programmed path, and high level motion commands, the machine""s internal numeric representation of the data required to interpolate the machine along the desired path to the end point of the current block.
The interpolation mode comprises a specification of the geometric path to be traversed by the grinding wheel from the programmed start point to the programmed end point.
An interpolated position command is a xe2x80x9csnap-shotxe2x80x9d of the desired machine position at some position on the programmed path. One interpolated position command is generated by the trajectory interpolator once every machine update period.
The position controller of the CNC machine conventionally receives the interpolated position commands every machine update period and performs a control algorithm that causes the cutting tool to attempt to attain the position determined from the interpolated position command.
Conventional CNC tool grinding machines have the workpiece held by a collet with-the centre of the workpiece in line with the rotational axis of the headstock. This method is subject to inaccuracy when aligning the centre of the workpiece with the rotational axis of the headstock. Accurate alignment is essential for concentricity of the ground workpiece.
The inability to set up for workpieces of varying diameters without changing or adjusting the machine""s collect reduces the flexibility and productivity of the conventional CNC tool grinding machine. The time spend on setup is unproductive and therefore undesirable.
A further limitation of the traditional collect method is limited rigidity of the length of the workpiece when subjected to the grinding means.
In order to reduce setup time and increase setup accuracy several types of workpiece clamps have been proposed.
One such mounting means which has been proposed for holding and rotating a workpiece is known alternatively as axe2x80x9csidelock holderxe2x80x9d or an xe2x80x9cend mill holderxe2x80x9d. Such a holder assembly is fitted to the collet. The workpiece is fitted within the end of the holder. The holder assembly requires a different sized mechanism to be fitted for each different sized workpiece. This method also requires manual adjustment. The holder provides greater rigidity than conventional mounting means but the need to fit a different assembly for varying workpiece sizes and manual adjustment requirement increase set up time. As such, the holder is not very practical in a CNC machine. Further, the holder does not address the problem of accurately setting up the centre of the workpiece to coincide with the centre of the headstock.
It is therefore desirable to provide a multi-axis CNC machine including workpiece mounting assembly which is adapted to hold workpieces of a range of diameters, which provides rigid support for the workpiece and which enables concentric grinding with minimal set up time.
It is also desirable to provide a multi-axis CNC machine, and a method of operating a CNC machine, which does not require accurate alignment of the centre of the workpiece with the centre of the headstock and therefore reduces setup time.
According to one aspect of the invention there is provided a method of controlling a CNC machine having workpiece mounting means for holding a workpiece and a cutting tool operable upon the workpiece under the control of the CNC machine, the workpiece mounting means being rotatable about a first axis, the method being characterised by the steps of:
clamping the workpiece such that a longitudinal axis of the workpiece is parallel to the first axis and offset from the first axis by a known or calculated distance; and
programming the CNC machine to compensate for said offset during operation of the cutting tool on the workpiece.
According to another aspect of the invention there is provided a computer numerically controlled (CNC) machine having workpiece mounting means for holding and rotating a workpiece, and a cutting tool operable upon the workpiece under the control of the CNC machine wherein the workpiece mounting means is rotatable about a first axis and includes workpiece clamping means for clamping workpieces of different transverse dimensions in such a manner that a workpiece is clamped with a longitudinal axis of the clamped workpiece parallel to the first axis, the offset between the first axis and the longitudinal axis of the workpiece being known or calculated, and wherein the CNC machine is programmed to compensate for said offset during operation of the cutting tool upon the workpiece.
The present invention is particularly applicable to CNC grinding machines for operating on workpieces having at least a cylindrical mounting portion. When such a workpiece is clamped with the central longitudinal axis of the cylindrical mounting portion extending parallel to, but offset from, the first axis, the workpiece revolves in an orbit around the first axis and, with the amount of offset being known or calculated, it is a relatively simple matter to program the CNC machine to compensate for the offset when controlling movement of the cuffing tool.
Preferably, the workpiece clamping means comprises a first clampingjaw that is fixed relative to the first axis about which the mounting means is rotatable and a second clamping jaw that is movable towards and away from the first clamping jaw. This is in contrast to rotatable chucks or collets of conventional CNC grinding machines which have a plurality of workpiece clamping jaws, each of which is movable relative to the axis of rotation of the chuck or collet.
One of the advantages of the present invention over conventional CNC grinding machines is that the workpiece clamping means can accommodate workpieces of different diameters without having to align the central longitudinal axis of each different diameter workpiece to coincide with the axis of rotation of the headstock, chuck or collet. Instead, the offset for each different diameter workpiece is already known, or can be readily calculated, and so the control program for the cutting tool of the CNC machine can automatically compensate for the offset of a particular diameter workpiece. This can result in a marked reduction in set up time when changing workpieces as opposed to conventional CNC grinding machines. Another advantage of the present invention is an increased concentricity of a ground workpiece, as inaccuracies in conventional CNC grinding machines caused by aligning the central axis of the workpiece with the rotational axis of the headstock are eliminated.
In a particularly preferred embodiment, the fixed clamping jaw of the clamping means is in the form of a V-block, having planar workpiece supporting surfaces extending at an angle relative to one another. Such a V-block provides rigid support for a clamped workpiece. Further, a V-block can be accurately formed with the planar workpiece supporting surfaces extending parallel to the axis of rotation of the mounting means. The movable clamping jaw of the clamping means may be moved towards and away from the fixed clamping jaw by any convenient operating mechanism. In one preferred arrangement, the movable clamping jaw is mounted on a pivotal lever arm. The lever arm may be conveniently pivoted by a pneumatic operating mechanism including a plunger which is engageable with a rear end of the lever arm.